#!/usr/bin/perl
use strict;
use warnings;
use DateTime::Format::Strptime;
use List::Util qw/ maxstr minstr /; # first released with perl v5.7.3

my $d1_beg = '2012-08-17 12:00:00';
my $d1_end = '2012-08-17 13:00:00';

my $d2_beg = '2012-08-17 09:00:00';
my $d2_end = '2012-08-17 12:30:00';

if ($d2_beg ge $d1_end or $d2_end le $d1_beg) {
    print "No common time\n";   
}
else {
    my $start = maxstr($d1_beg, $d2_beg);
    my $end   = minstr($d1_end, $d2_end);
    
    my $dt = DateTime::Format::Strptime->new(pattern => "%F %T");
    
    my @dt = map $dt->parse_datetime($_), $start, $end;
    my $dur = $dt[1]->delta_ms($dt[0])->in_units('minutes'); # duration
    
    print "$start for $dur minutes\n";
}

__END__
*** prints
2012-08-17 12:00:00 for 30 minutes